An important function that must be provided in high quality optical networks is that of wavelength multiplexing and demultiplexing. In particular, channel adding/dropping filters are needed at each node to combine and separate different wavelength channels. Typically the filter must have rectangular wavelength response, with negligible loss in the passbands, and high rejection in the stopbands.
These properties are currently realized in two stages as in FIG. 1 where the first stage is a periodic slicer with maximally flat passbands. The slicer separates 2N input channels, centered at the wavelengthsλ1, λ2, λ3, λ4, . . . , λ2N−1, λ2N
into two interleaved sets, centered at λ1, λ3, . . . , λ2N−1 and λ2, λ4, . . . , λ2N, respectively including the even and odd wavelengths. Since the N wavelengths of either set are widely spaced (by twice the input channel spacing) they can be easily separated by a conventional waveguide grating router (which will be simply called a grating). Advantages of this technique are 1) reduced crosstalk between even and odd channels (since two stages of filtering are involved) and 2) reduced loss variation in each passband. The first property is particularly important since it reduces a major source of crosstalk, namely between adjacent channels λj and λj−1. The second property is obtained because the passbands in the first stage are maximally flat and, in the second stage, they are widely spaced. Note, however, the complete arrangement is not characterized by maximally flat passbands.
In the above arrangement, the loss variation in the vicinity of each wavelength of maximum transmission is primarily caused, in each output router, by the wavelength dependence of each output image produced by the grating. Because of this wavelength dependence, each output image is generally displaced from the intended output waveguide location. This displacement is the primary cause for the above loss variation, and it can be essentially eliminated by using a planar arrangement of two gratings of opposite dispersions as shown previously in U.S. Pat. No. 5,488,680, which issued on January 1996, and U.S. Pat. No. 7,003,198 B2, which issued on February 2006. This technique is illustrated in FIG. 2, showing schematically a planar arrangement of two waveguide grating routers 201 and 202 and a waveguide lens 203 connected between the two gratings. In this arrangement, an input signal of variable wavelength applied to the input waveguide 204 is transformed by the first router 201 into a variable image B1 (206) produced inside the principal zone P1P2 of the router. Thus, if the image 206 is produced inside the lens aperture, it is transferred efficiently by the lens to the second router 202, which in turn produces an output image B3 (209), whose wavelength dependence is primarily determined by the dispersion coefficients of the two gratings producing B3. Here, since the two gratings have opposite dispersions, the image B3 is essentially stationary, and it can be received efficiently by an output waveguide 210 located at the (stationary) image location. This arrangement, therefore, has substantially reduced loss variation in the vicinity of each wavelength of maximum transmission, thus featuring to a good approximation maximally flat passbands.
A disadvantage of the technique of FIG. 2 is the large size of each grating as compared to a conventional grating. On the other hand, the increased size (the increased number of arms in each grating) can be shown to result in lower crosstalk. By using this technique, an efficient 1×N router with low values of loss and crosstalk has been realized by including between the two waveguide grating routers N lenses respectively producing N stationary images at the locations of the N output waveguides. Notice only one of the N lenses is shown in FIG. 2.
In the present application an efficient 1×N router is realized by a simpler technique, which makes use of the cyclic behavior of B1 in the principal zone P1P2 of the input grating. As shown in FIG. 3, now only one lens 303 is used between the two gratings 301 and 302. The lens aperture now essentially covers the entire principal zone P1P2 (indicated as 307) of the input grating, and therefore now each cycle of B1 306 produces one stationary image B3309, and N cycles are needed in order to produce N stationary images. The new arrangement is simpler to realize, since it includes only one lens, and the size of the input grating is now substantially reduced. On the other hand, since the lens aperture now essentially covers the entire principal zone 307, a full cycle of the image 306 is transmitted to each output waveguide. Therefore, in order to obtain a maximally flat response in the output waveguide (only one output waveguide 310 is shown) the efficiency variation of the first grating must be minimized over a substantial part of the principal zone 307.
In order to optimize the efficiency of the arrangement in FIG. 3, one must optimize the efficiencies of the two gratings and the lens. In each case the grating (or the lens) is formed between two periodic arrays (of radial waveguides) which determine the input and output efficiency of the grating or the lens. Therefore the complete arrangement includes a total of six periodic arrays. The total efficiency is the product of six efficiencies, each contributed by a particular array, and it is important to minimize the total loss by including in each array suitable segmented matching transitions as disclosed in U.S. Pat. No. 7,068,888 B1, which issued on Jun. 27, 2006. Consider for instance the efficiency of power transfer from the first grating to the lens. In this case one must optimize the arrangement of FIG. 4, consisting of two periodic arrays (each including two matching sections) separated by a slab region. Once the efficiency is optimized, one finds that the waveguides of each array are strongly coupled and therefore, in accordance with U.S. Pat. No. 5,136,671, which issued on Aug. 4, 1992, the phase center of each array is displaced from the edge of the slab. One also finds that aberrations are minimized by letting the phase center of each array coincide with the focal point of the radial waveguides of the other array. Residual aberrations are then small, and can be substantially eliminated by including suitable corrections in the waveguides of the lens and the grating (connected to the arrangement of FIG. 4). Similar considerations apply to the other periodic arrays. Also notice that an important design requirement is that the total transmission loss must be about the same for all output waveguides. Therefore, since the loss variation in this case is caused by the output array of the output router, it is important to optimize this array. In this case, since the output array has a long focal lens, excellent performance can be realized by using (in addition to suitable matching sections) the technique described in U.S. Pat. No. 6,873,766 B2, which issued on Mar. 29, 2005. The array loss will then be typically negligible over at least 80% of the array central zone (thus insuring excellent uniformity for the loss of different output waveguides). In the following drawings, all matching sections will be omitted for simplicity, and the drawings will be further simplified by omitting in each case the phase center displacement.
To summarize, the arrangement of FIG. 1 is difficult to realize in integrated form, and it is afflicted by substantial loss variation in each passband. This variation is substantially eliminated by using the planar arrangement of FIG. 3, which can be realized in integrated form on a single wafer, and it is simpler than the prior art arrangement (obtained by including N lenses in FIG. 2) since the new arrangement requires only one lens between the two gratings, and the size of the input grating is substantially reduced. An additional advantage of the arrangement of FIG. 3 is that crosstalk can now be further reduced by including two gratings in the output stage as shown in FIG. 5. As shown in FIG. 5, the new arrangement is similar to FIG. 1, except that 1) the first stage is now realized by using a grating and 2) a composite lens is now used between the two stages. The two stages are again characterized (as in FIG. 2) by opposite dispersions and, therefore, they perform stationary imaging. In the arrangements of FIGS. 3, 5 one finds that the loss variation in each passband is primarily determined by the output efficiency of the first grating and therefore, by including suitable matching sections at the output of the first grating, the arrangement is (approximately) characterized by maximally flat response in each passband. Moreover, in both arrangements, adjacent channels crosstalk is substantially reduced, as compared to a conventional waveguide grating router.